Step 1: In a large mixing bowl, combine the softened unsalted butter, creamy peanut butter, and vanilla extract. Beat with an electric mixer on medium speed until light and fluffy, about 2-3 minutes, scraping down the sides of the bowl as needed.
Step 2: Gradually add the sifted powdered sugar to the peanut butter mixture, ½ cup at a time, mixing on low speed until fully incorporated after each addition. Increase speed to medium and beat until the mixture forms a stiff, yet pliable dough.
Step 3: Line a large baking sheet with parchment paper or wax paper. Using a small cookie scoop or a tablespoon, roll the peanut butter mixture into smooth, uniform 1-inch balls. Place them on the prepared baking sheet.
Step 4: Insert a toothpick into the top of each peanut butter ball, leaving enough room at the bottom for dipping. Chill the baking sheet in the refrigerator for at least 30-60 minutes, or in the freezer for 15-20 minutes, until firm.
Step 5: In a microwave-safe bowl, combine the semi-sweet chocolate chips, milk chocolate chips, and vegetable shortening (if using). Microwave in 30-second intervals, stirring well after each, until completely melted and smooth. Be careful not to overheat.
Step 6: Remove the chilled peanut butter balls from the refrigerator/freezer. Holding a ball by its toothpick, dip it into the melted chocolate, leaving a small circle of peanut butter exposed at the top. Allow excess chocolate to drip off.
Step 7: Carefully place the dipped buckeyes back onto the parchment-lined baking sheet. Once all are dipped, refrigerate for another 15-30 minutes, or until the chocolate is completely set. Gently remove toothpicks and smooth over any holes if desired. Store in an airtight container in the refrigerator for up to 2 weeks.